The invention relates to a method for evacuating transfer air contained in a binding agent from a mixture of pressurized air and binding agent when stabilizing earth masses by adding binding agent, in which method is used an apparatus comprising means for producing pressurized air, a binding agent container, a supply pipe for the mixture of pressurized air and binding agent, and an apparatus for mixing the binding agent into the earth mass. In the method, the pressurized air used for transferring the binding agent is evacuated through a separate discharge pipe by means of the following measures before the pressurized air in conveyed into the earth mass: 1) the binding agent is conveyed into a storage space with a binding agent discharge opening into the ground, and 2) the pressure level in the apparatus is adjusted to be such that the pressure in the storage space exceeds the counter-pressure caused by the ground at the discharge opening, whereupon the binding agent discharges from the storage space via the discharge opening, out into the ground, and at least a part of the air discharges controllably from the upper part of the storage space into the discharge pipe.
Commonly used stabilization methods can be divided into two basic methods. In Japan a widely used method is when a binding agent, mainly cement, mixed in water, is supplied into the ground. The binding agent is mixed in large units and delivered to the worksite in a form ready to be supplied into the ground. This system makes the dosing of the binding agent highly accurate because the supply of liquid can be cut off and started without the slowness of an air supply. The supply units are also so-called non-pressurized containers which contain a pump unit and thus the technology is simple. The problem with this wet method is its susceptibility to external malfunctions, that is, the supplies of binding agent must be accurately timed and there may not be very serious malfunctions in the mixing and feeding devices themselves in order for the hardening binding agent not to clog inside the devices.
The other basic method is a method developed in the Nordic Countries, wherein the binding agent is delivered to the worksite in powder form and is transferred in pressurized form into storage containers, and from there further in pressurized form into supply containers. The pressure in the storage and transfer containers is usually 1-2 bars and in the supply containers 6-8 bars. In this method, the binding agent is supplied and dosed into the pressurized air in dry form, the advantage then being that the logistics of the binding agent deliveries are not very strict, as long as there is always binding agent in the storage container at the worksite. The binding agent also remains usable for a long time in powder form and thus sudden work stoppages will not cause problems to the feeding apparatuses.
In both methods, the binding agent is supplied and mixed into the ground by means of a mixing head fixed to the end of a rotating pipe. There may be several of these rotating pipes connected into a group, whereby several pillars can be made in one work stage. The rotating pipe may be round or a polygon, usually square, inside which the binding agent is conveyed into the ground.
Since the entire pipe with the mixing head at its lower end rotates, when going deeper, a major part of the rotating torque is required for other than rotating the actual mixing head and for mixing the soil. The rotating torque is usually transferred to the pipe by means of a transmission chain or gear transmission fixed to its upper end.
In all methods, the actual pillar stabilization unit is a large and heavy device, the moving of which from one worksite to another is slow and expensive. Due to its massiveness, the device itself is expensive in terms of investment costs.
Advantages of the wet method are an accurate supply of binding agent and minor interference with the surrounding soil. A disadvantage is the inapplicability of the method to sites where the natural water content of the soil to be treated is high (e.g. most clays in Scandinavia). At these worksites, the quality of the pillar is impaired by the fact that the pre-mixed mixture of binding agent and water does not mix well in the ground, but tends to penetrate to the surface, thus causing quality variations in the pillar. As disadvantages may also be considered the distance of the production site of the binding agent from the worksite and the logistic problems caused by this and the limited time between the production of the binding agent and feeding it into the ground, which does not allow for interference or stoppages in the process.
Advantages of the dry method are a greater independence of the supplier of the binding agent and the storability of the binding agent at the worksite, which allows for more flexible working. The disadvantages include the conveyance of pressurized air into the ground, where it interferes with the surrounding soil and impairs the quality of the pillar, and due to the different layers of the soil, a part of the binding agent is discharged from the pillar through pressure discharge channels. Dusting is also sometimes a problem, although with appropriate work methods it can be almost completely eliminated. Interference with the surrounding soil affects the bearing capacity of the pillar and the amount of binding agent in the pillar may vary greatly over a short distance depending on the porosity of the soil.